Biotic Communities of the Colorado Plateau

Mixed-Conifer Forest

Dense mixed-conifer forest. Under a natural fire regime, these forests are often more open. Photo by Keith Pohs.

On the Colorado Plateau mixed-conifer forests generally occur at elevations from about 8000 feet to 10,000 feet, where annual precipitation is from 25 to 30 inches annually. In contrast to the ponderosa pine forest growing at slightly lower elevations, the mixed-conifer forest is comparatively lush and much more diverse. Depending on location, Douglas-fir, white fir, limber pine (in the north), blue spruce, and less commonly southwestern white pine form mixed stands in this community, with ponderosa pine joining the mix on warmer slopes. Blue spruce is common in moist areas on many of the high plateaus of central and southern Utah, while limber pine is a major component of mixed-conifer forests on the San Francisco Peaks near Flagstaff, Arizona. Quaking aspen, along with Gambel oak, is prominent in these forests following disturbances.

Under natural conditions the relatively open nature of the mixed-conifer forest coupled with the rich, moist soil found at these elevations allows for the development of a diverse understory of forbs, grasses, and shrubs. Understory conditions vary widely, from dry, open-canopy forests with grassy undergrowth on open slopes and ridges to moist, closed-canopied stands dominated by numerous herbaceous plants in the canyons and ravines. A popular zone for wildlife due to the abundant forage, black bears, mule deer, and elk can be quite common.

Fire histories in mixed-conifer forests vary with forest composition, landscape characteristics and human intervention. Until the late 1800s when grazing and fire suppression policies vastly reduced widespread fires, fire regimes included frequent surface fires to infrequent, patchy crown fires with return intervals of about 10 years. In the absense of fire these forests have undergone major changes in structure and species composition throughout the American west.

Ponderosa pine was once codominant in many mixed-conifer forests with relatively open stand structures, but fire suppression has allowed the development of dense sapling understories, with regeneration dominated by the more fire-sensitive Douglas-fir and white fir. Forest stand inventory data from Arizona and New Mexico show an 81% increase in the area of mixed-conifer forests between 1962 and 1986. Herbaceous understories have been reduced by denser canopies and needle litter, and nutrient cycles have been disrupted. Heavy surface fuels and a vertically continuous ladder of dead branches have developed, resulting in increased risks of crown fires. As these forests become reduced or fragmented, local endemic plants may become threatened or endangered.

Research:

Changed Southwestern Forests: Resource effects and management remedies. Over 150 years of occupancy by northern Europeans has markedly changed vegetative conditions in the Southwest. Less fire due to grazing and fire suppression triggered a shift to forests with very high tree densities, which in turn contributed to destructive forest fires. Options to deal with these changes include prescribed fire, thinning and timber harvest to mimic natural disturbances and conditions. However, there are barriers to implementing these activities on a scale large enough to have a significant benefit. Adapted from a published journal article by Marlin Johnson.

Late Holocene Environmental Change in the Upper Gunnison Basin, Colorado. The Upper Gunnison Basin is a high elevation (3100 to 3600 m) region on the edge of the Colorado Plateau in southwestern Colorado. Its unusual ecological characteristics include an absence of plant and animal taxa that should occur here. Fossil and archaeological evidence indicates that many of the missing species existed in the Basin during the late Pleistocene to middle Holocene. Authored by Steve Emslie.

Bennett, P. S. 1974. The ecological role of fire in North Rim forests, Grand Canyon National Park. Plateau 46: 168-181.

Dieterich, J. H. 1983. Fire history of southwestern mixed-conifer: a case study. Journal of Forest Ecology and Management. 6: 13-31.

Fitzhugh, E. L., Moir, W. H., Ludwig, J. A. and Ronco, F., Jr. 1987. Forest habitat types in the Apache, Gila, and part of the Cibola National Forests, Arizona and New Mexico. General Technical Report RM-145. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, 116 pp.

Johnson, M. 1994. Changes in Southwestern forests: Stewardship implications. Journal of Forestry 92: 16-19.

Jones, T. R. 1974. Silviculture of southwestern mixed-conifers and aspen: The status of our knowledge. Report RM–122. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, 44 pp.

Kaufmann, M. R., Moir, W. H. and Covington, W. W. 1992. The status of knowledge of old-growth forest ecology and management in the central and southern Rocky Mountains and Southwest. Pp. 231-277 In: Mooney, H. A., Bonnicksen, T. M., Christensen, N. L., Lotan, J. E. and Reiners, W. A., editors. Old-growth forests in the Southwest and Rocky Mountain regions. General Technical Report RM-213. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.

McHenry, D. E. 1933. Woodland parks on the North Rim. Grand Canyon Nature Notes 8: 195-198.

Moir, W. H. and Ludwig, J. A. 1979. A classification of spruce–fir and mixed-conifer habitat types in Arizona and New Mexico. Research Paper RM–207. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, 47 pp.

Moore, M. M. 1994. Tree encroachment on meadows of the North Rim of Grand Canyon National Park. Report #CA B000-B-0002. National Park Service, 86 pp.

Savage, M. 1997. The role of anthropogenic influences in a mixed-conifer forest mortality episode. Journal of Vegetation Science 8: 95.

Stein, S. J. 1988. Explanations of the imbalanced age structure and scattered distribution of ponderosa pine within a high-elevation mixed-coniferous forest. Forest Ecology and Management 25: 139-153.

Touchan, R., Allen, C. D. and Swetnam, T. W. 1996. Fire history and climatic pattens in ponderosa pine and mixed-conifer forests of the Jemez Mountains, northern New Mexico. Pp. 179-195 In: C.D. Allen, editor. Fire Effects in Southwestern forests: Proceedings of the Second La Mesa Fire Symposium. USDA Forest Service General Technical Report RM-286, Fort Collins, CO.

USDA Forest Service. 1993. Changing conditions in southwestern forests and implications on land stewardship. U.S. Forest Service, Southwest Region, Albuquerque, N.M., 8 pp.